1. Field of the Invention
The present invention relates to an image forming apparatus and a droplet ejection control method, and more particularly to a recording control technology for an image forming apparatus and a droplet ejection control method for forming an image by means of dots formed with organic pigment ink on a recording medium.
2. Description of the Related Art
Recently, inkjet recording apparatuses (inkjet printers) have become common as recording apparatuses for printing and recording images captured by digital still cameras, and the like. An inkjet recording apparatus comprises a plurality of recording elements (nozzles) in a head, the recording head being move to scan a recording medium while droplets of ink are ejected onto the recording medium from the recording elements, the recording medium being conveyed through a distance corresponding to one line, each time one line of an image is recorded onto a recording medium, and an image being formed onto the recording paper by repeating this process.
Inkjet printers include those which use a fixed-length serial head, and carry out recording by moving the head in the lateral direction of a recording medium, and those which use a line head in which recording elements are arranged over a length corresponding to the full dimension of one edge of the recording medium. In a printer using a line head, it is possible to record an image across the entire surface of the recording medium, by scanning the recording medium in an orthogonal direction to the direction in which the recording elements are arranged. In a printer using a line head, it is not necessary to provide a conveyance system, such as a carriage, for moving a short-dimension head, nor is it necessary to move a carriage, or perform complicated scanning control of the recording medium. Furthermore, since only the recording medium is moved, it is possible to increase the recording speed in comparison to printers using serial heads.
In an inkjet printer, one image is represented by combining dots formed by ink ejected from recording elements (nozzles). High image quality can be achieved by making the dots small in size and by using a large number of pixels per image. Small dot size can be achieved by reducing the amount of ink ejected, and therefore it is necessary to control the ink ejection volume finely and accurately. The relative speed of the recording medium and the recording head, and the ink ejection timing, are controlled in such a manner that adjacently positioned dots are deposited at prescribed positions.
Japanese Patent Application Publication No. 3-247450 discloses an inkjet recording apparatus and proposes a technology for calculating printing times for avoiding image distortion, according to the ink absorption characteristics, ink permeability, ink (dot) density, ink volume, ink evaporation characteristics, and ambient temperature. In other words, the drying time of the ink, and the like, is estimated from the aforementioned parameters, and the interval between one recording operation and the next is adjusted accordingly.
Furthermore, Japanese Patent Application Publication No. 10-250059 discloses a method of manufacturing a recording head for an inkjet printer, and a printing method; more specifically, a method of manufacturing a print head in which the distance between nozzles is determined in accordance with the ink drying time, and a print method using this print head.
On the other hand, inks to be used in inkjet printers are roughly classified into dye-based inks and pigment-based inks. As the coloring material for ink-jet ink, dyes have been used, but they have drawbacks as to water resistance and light resistance. To overcome the drawbacks, pigments have come to be used. Images obtained from pigment inks have remarkable advantages of superior light resistance and water resistance compared with images obtained from dye-based inks. However, there are problems that the pigment is not easily formed uniformly or pulverized into fine-particles of a nanometer size (i.e., monodispersion), which can permeate pores in the surface of paper, and it is then difficult to form images with high saturation because the pigment is poor in permeability into the paper.
As measures to solve the above-described problems, Japanese Patent Application Publication No. 2002-146222 discloses a method to produce fine particles of pigment using a microjet reactor method. This is a method of introducing a solution, in which a pigment is dissolved, and a precipitation medium solution, into two nozzles that are opposite to each other and that have different micrometer sizes, at high pressure (for example, 5 MPa) by means of pumps; and introducing a gas (such as compressed air) perpendicularly into a portion where the jet flows of the two solutions collide with each other, so as to discharge a suspension of the pigment by the gas flow (flow rate, about 0.5 m3/h).
In the image formation by means of the inkjet recording apparatus, there are problems that if a plurality of dots are ejected so as to land in an overlapping manner on a recording medium, the ink droplets combine, the original circular shapes of the dots are lost (i.e., occurrence of deposition interference), and hence it becomes difficult to form the desired image with high resolution.
However, it is difficult to solve the problems concerning the deposition interference with the measures disclosed in Japanese Patent Application Publication Nos. 3-247450 and 2002-146222. In particular, when the pigment ink containing the pigment particles of large particle size and poor monodispersion is used, it is difficult to form images with high saturation, and moreover, the deposition interference may be aggravated.
In the inkjet recording apparatus disclosed in Japanese Patent Application Publication No. 3-247450, the time for avoiding image distortion is determined for each parameter, but it is extremely difficult to determine a time for avoiding image distortion in relation to various types of dot patterns. Furthermore, no sufficient explanation of the recording method is given in cases where mixed patterns combining different dot pitches and dot sizes are formed in one image.
In the method for manufacturing a recording head for an inkjet printer and the printing method disclosed in Japanese Patent Application Publication No. 10-250059, it is stated that a second dot is ejected so as to land after the drying time for a first dot has elapsed. However, since an adjacent dot cannot be deposited onto the paper until the existing dot on the surface of the paper has dried completely, then printing takes an extremely long time. Furthermore, since the distance between the nozzles is determined by estimating the drying time based on one set of conditions, and since this is a fixed value, it is not possible to respond to cases where different ink or paper is used.
In the method for producing fine particles of pigment disclosed in Japanese Patent Application Publication No. 2002-146222, particles are generated in a very small space, of a micrometer scale, and the particles are immediately taken out from the apparatus, thereby preventing the apparatus from being blocked by the pigment fine-particles. This method is then preferred to obtain fine particles having a narrow particle size distribution. However, the method has such problems as that the contact time for the two solutions is not easily controlled, and thus subtle reaction control is difficult. Therefore, it is difficult to stably produce fine particles of pigment of sufficient monodispersion.
In the related art as described above, there are unsolved problems of prevention of the deposition interference and production of fine particles of pigment, and it is hence difficult to obtain images of high resolution and high saturation by means of the pigment ink.